Include kernel.h from configfs.h

configfs.h uses the container_of macro and as such should include kernel.h.

Signed-off-by: Ben Nizette <bn@niasdigital.com>
Cc: Joel Becker <Joel.Becker@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>